1. Field of the Invention
The present invention relates to a technology for correcting a local time of a time-stamping device that affixes a digital signature including therein the local time to an electronic document.
2. Description of the Related Art
With the recent developments in the field of electronic authentication technology, digital signatures that authenticate a creator or publisher of electronic documents have come to be widely used. The digital signature uses technology such as encryption key, etc. to enhance its reliability. Further, attempts have been made to include the national standard time (hereinafter, “standard time”) in the digital signature to authenticate the creation time or transmission time of the electronic document.
A device that affixes a digital signature with a time stamp is generally known as a time-stamping device. The time-stamping device has an internal clock. As well as clocking the local time according to the internal clock, the time-stamping device also corrects the local time by receiving radio waves that include the standard time, thereby enhancing the accuracy of the time stamped in the digital signature.
To affix a digital signature with a time stamp, it is essential to keep the difference between the local time of the time-stamping device and the standard time within a predetermined threshold value. That is, by ensuring that the difference between the time included in the digital signal and the standard time is kept within the predetermined threshold value, the time stamp of the electronic document that is to be digitally signed can be authenticated.
One method that may be employed for keeping the difference between the local time and the standard time within the predetermined threshold level is by receiving the radio wave, as described earlier. Another method is by connecting to a standard time managing server connected to a network and obtaining the standard time from the server. For instance, the standard time managing server disclosed in Japanese Patent Laid-Open Publication No. 2002-229869 transmits the standard time with an expiration data to a client device that is constantly connected to the server, and detects any deviation or tampering with the internal clock of the client device.
However, in the conventional time-stamping device fraudulent falsification of the local time cannot be prevented. For instance, the local time of the time-stamping device can be manipulated to be much ahead of or behind the authentic time with the aid of a radio wave including therein a false standard time instead of the true standard time. Thus, the time stamp on the electronic document cannot be authenticated with this kind of doctored local time.
Even if a mechanism is provided that monitors the difference between the local time of the time-stamping device and the standard time included in the radio wave and determines that the clock has been tampered with if the difference exceeds the predetermined threshold value, the mechanism will not be able to detect falsification of the local time when there is a combined influence of exposure of the time-stamping device to either a high temperature or a low temperature and a false radio wave.
Further, with the public preference for compact devices, the need of the hour is a compact time-stamping device that does not require to be connected all the time to a network, such as a local area network (LAN), and that can be carried around like a wrist watch or a mobile, and used whenever required.
The conventional technology disclosed in Japanese Patent Laid-Open Publication No. 2002-229869 cannot be applied to a compact time-stamping device since the technology needs the client device to be constantly connected to the network such as the LAN so that it can always be in communication with the standard time managing server.
Thus, it is important to realize a time-stamping device that enhances the reliability of the time included in a digital signal by preventing fraudulent falsification of the time stamp as well as guarantees the authenticity of the time stamp without having to be constantly connected to a network.